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Java Threads

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Java Threads What is a Thread? A thread can be loosely defined as a separate stream of execution that takes place simultaneously with and independently of everything ... – PowerPoint PPT presentation

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Title: Java Threads


1
Java Threads
2
What is a Thread?
  • A thread can be loosely defined as a separate
    stream of execution that takes place
    simultaneously with and independently of
    everything else that might be happening.
  • A thread is like a classic program that starts at
    point A and executes until it reaches point B
  • A thread runs independently of anything else
    happening in the computer.
  • Without threads an entire program can be held up
    by one CPU intensive task or one infinite loop,
    intentional or otherwise.
  • With threads the other tasks that don't get stuck
    in the loop can continue processing without
    waiting for the stuck task to finish.

3
  • It turns out that implementing threading is
    harder than implementing multitasking in an
    operating system.
  • The reason it's relatively easy to implement
    multitasking is that individual programs are
    isolated from each other.
  • Individual threads, however, are not.

4
  • Threaded environments like Java allow a thread to
    put locks on shared resources so that while one
    thread is using data no other thread can touch
    that data.
  • This is done with synchronization.
  • Synchronization should be used sparingly since
    the purpose of threading is defeated if the
    entire system gets stopped waiting for a lock to
    be released.
  • The proper choice of objects and methods to
    synchronize is one of the more difficult things
    to learn about threaded programming.

5
How Java Uses Threads
  • Java applications and applets are naturally
    threaded.
  • The runtime environment starts execution of the
    program with the main() method in one thread.
  • Garbage collection takes place in another thread.
  • Screen updating occurs in a third thread.
  • There may be other threads running as well,
    mostly related to the behavior of the applet
    viewer or web browser.
  • All of this happens invisibly to the programmer.

6
When to thread
  • The simplest reason for adding a separate thread
    is to perform a long calculation.
  • For instance if you're trying to find the ten
    millionth prime number, you probably don't want
    to make users twiddle their thumbs while you
    search.
  • Or you may be waiting for a resource that isn't
    available yet, a large graphic to download from
    the Internet, for example.
  • Once again you shouldn't make the user wait while
    your program waits.
  • Any operation that is going to take a noticeable
    period of time should be placed in its own
    thread.

7
  • The other reason to use threading is to more
    evenly divide the computer's power among
    different tasks.
  • If you want to draw random rectangles on the
    display, you would still like the applet to
    respond to user input.
  • If all the CPU time is spent drawing rectangles,
    there's nothing left over for the user.
  • On a preemptively multitasking operating system
    like Solaris or Windows NT, the user may at least
    be able to kill the application.
  • On a cooperatively multitasking operating system
    like the MacOS or Windows, the user may have to
    reboot their machine.
  • This is a bad thing.
  • With threads you can set the priority of
    different processes, so that user input receives
    a high priority and drawing pretty pictures
    receives a low priority. Then the user can stop
    the applet without flipping the power switch on
    their machine.

8
The Thread Classes
  • The Thread class has three primary methods that
    are used to control a thread
  • public void start()
  • The start() method prepares a thread to be run
  • public void run()
  • the run() method actually performs the work of
    the thread
  • public final void stop()
  • the stop() method halts the thread.
  • The thread dies when the the run() method
    terminates or when the thread's stop() method is
    invoked.

9
  • You never call run() explicitly.
  • It is called automatically by the runtime as
    necessary once you've called start(). There are
    also methods to supend and resume threads, to put
    threads to sleep and wake them up, to yield
    control to other threads, and many more.

10
Runnable Interface
  • The Runnable interface allows you to add
    threading to a class which, for one reason or
    another, cannot conveniently extend Thread. It
    declares a single method, run()
  • public abstract void run()
  • By passing an object which implements Runnable to
    a Thread() constructor, you can substitute the
    Runnable's run() method for the Thread's own
    run() method. (More properly the Thread object's
    run() method simply calls the Runnable's run()
    method.)

11
  • To use threads efficiently and without errors you
    must understand various aspects of threads and
    the Java runtime system.
  • You need to know how to provide a body for a
    thread,
  • the life cycle of a thread,
  • how the runtime system schedules threads, thread
    groups,
  • and what daemon threads are
  • and how to write them.

12
Threads
  • Three fields
  • Max_Priority
  • Min_Priority
  • Normal_Priority

13
Static Thread methods
  • Thread th Thread.currentThread()
  • gets the Thread object for the thread that
    executes this call (the "current thread").
  • Thread.sleep(milliseconds)
  • puts the current thread to sleep for that length
    of time. (almost always called in a try block to
    catch the InterruptedException.)
  • Thread.yield()
  • lets another thread run (if there is one that can
    run).

14
normal Thread methods
  • void start()
  • starts the thread running. The run() method of
    the Runnable attached to the Thread is called in
    that thread.
  • void stop()
  • stops the thread immediately. The thread cannot
    be restarted.
  • void suspend()
  • stops the thread from running, until a resume()
    is called.

15
  • void resume()
  • causes the thread to continue running from where
    it was suspended.
  • void setPriority(int prio)
  • changes the priority of the thread. Threads with
    a higher valued priority run preferentially to
    lower priority threads.
  • int getPriority()
  • returns the priority of the thread.
  • void join()
  • causes the current thread to wait until this
    thread is finished (either run() returns or the
    thread is stop()-ed).

16
Daemon Threads
  • User Threads
  • Daemon Threads
  • When a Java Virtual Machine starts up, there is
    usually a single non-daemon thread (which
    typically calls the method named main of some
    designated class). The Java Virtual Machine
    continues to execute threads until either of the
    following occurs
  • The exit method of class Runtime has been called
    and the security manager has permitted the exit
    operation to take place.
  • All threads that are not daemon threads have
    died, either by returning from the call to the
    run method or by throwing an exception that
    propagates beyond the run method.
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